The present invention relates generally to disc refiners for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually have radial bars and grooves. A large volume of steam is produced between the plates as a result of this retained between the plates on the bar surfaces despite the high velocity of the flowing steam, and the enormous centrifugal forces. Typically, the steam is exhausted via the grooves. Dams which extend to the bar surface are provided in the grooves to interrupt material flow and thus improve the retention time of the material in the refining region.
In a typical refiner plate with radial bars and grooves, the bars provide impacts or pressure pulses which separate and fibrillate the fibers. The grooves enable radially directed feeding of the fibers and steam extraction. Near the perimeter of the plates, high radial steam flow and high centrifugal force both act to sweep the fibers outwardly from between the plates prematurely, thus reducing the refining effectiveness. The flow restrictions due to a small gap between opposed plates and fiber-filled grooves result in a steam pressure peak between the plates, located radially inward from the perimeter. This pressure peak is a major source of the refining thrust load, and can induce control instability at high motor loads. It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while retaining the pulp within the region as long as possible.
Although the surface dams promote better and more homogeneous refining and help develop the pulp pad between the segments, they restrict the flow of steam in the grooves. Reduced steam flow results in higher pressure peaks, creating a greater thrust load and more instability.
Briefly stated, the invention is a refiner plate segment for refining lignocellulosic material. The refiner plate segment has a refining zone comprising a plurality of alternating, substantially radially disposed bars and grooves. Each of the bars has a top grinding surface, a leading surface and a trailing surface. At least one dam extends radially across a groove intermediate the leading bar and the trailing bar of a bar pair. The dam comprises a first portion extending from the trailing surface of the leading bar and a second portion extending from the leading surface of the trailing bar. Each of the portions of the dam have a top surface, where the top surface of the second portion is substantially co-planar with the top surface of the bars and the top surface of the first portion is disposed intermediate the top surface of the bars and the base of the groove.
Preferably, a plurality of the dams are disposed in each of the grooves. In one embodiment, the trailing surface and the top surface of the leading bar define a cavity disposed adjacent the first portion of the dam. The cavity creates an arcuate shaped segment in the trailing face of the leading bar and has a bottom which is substantially co-planar with the top surface of the first portion of the dam to reduce the possibility of inducing turbulence in the flow of material.
It is an object of the present invention to provide a refiner plate for the face of a refiner disc, which facilitates the removal of steam while retaining the pulp in the refiner region to achieve satisfactory pulp quality.
It is also an object of the present invention to provide a refiner plate for the face of a refiner disc, which directs the material to be refined onto the refining surfaces of the disc.
Other objects and advantages of the invention will become apparent from the drawings and specification.